The thermal conductance of existing cold formed metal framing systems is a continuing concern in construction: particularly the construction of metallic framed structures. Metals, including those greatly used in construction such as iron and aluminum, are typically highly heat conductive. As heat is transferred and lost from the inside to the outside, or vice versa, through the heat-conductive studs themselves, the composite thermal resistance of the framing members and cavity insulation are reduced by approximately fifty percent.
As an example, the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) considers the effective R-value of R-19 fiberglass batt insulation in the stud cavity of a wall framed with 6 inch metal studs at 24 inches on center as R-8.55. The same wall using 2×6 wood studs would be considered as R-19.11 by ASHRAE. However, wood and other-energy-efficient framing materials are often unsuitable for commercial construction requiring incombustible materials and therefore metal framing must be used.
This has costly consequences for power consumption. Lower thermal resistance in an exterior wall assembly causes a building's HVAC systems to use more energy heating and/or cooling to keep the building occupants comfortable. Roughly 40% of the total USA energy consumption in the year 2012 was consumed in residential and commercial buildings. Over 30% of a building's energy consumption is for heating and cooling the building. The United States currently uses about 25 PWh (25 Petawatt-hours=25 billion megawatt-hours) per year which means that approximately 3 PWh of energy is used each year for building heating and cooling.
New requirements for green and energy-efficient buildings are making it more important than ever that these energy-efficiency and heat conductivity issues be addressed. The 2012 IECC (International Energy Conservation Code) has made significant changes to the required thermal resistance of exterior walls in commercial buildings. These changes have been brought in line with recommendation from ASHRAE standard 90.1 (2010 edition). As the 2012 IECC is adopted into law by cities and states for new commercial construction, new buildings will be required to meet increasingly stringent requirements. Meeting these requirements with exterior veneer systems which do not include continuous rigid insulation as part of the system will become more and more difficult.
EIFS (exterior insulation finish systems) is an example of an exterior veneer system with includes continuous rigid insulation as part of the system; limestone veneer is an example of an exterior veneer system which typically does not include continuous insulation as part of the system. Exteriors using continuous rigid insulation and no cavity insulation typically need to have an insulation thickness of at least six inches in order to be effective, which can result in exterior walls with a total thickness of 12 to 18 inches depending on the structural requirements. These walls are also limited to light-weight options for exterior veneers such as EIFS finishes due to the greater distance between the veneer and the structural framing.
These more stringent requirements are addressed to some degree by a method of construction wherein there an outer shell, often metallic, is supported by framing members protruding from a weather/air barrier often comprised of an inner frame, and lying between the inner and outer frame is a cavity, usually comprised of air, which acts as a heat transfer buffer. Architectural detailing currently calls for locating rigid insulation between the exterior veneer system and the internal weather/air barrier. However, the use of metal studs between the inner and outer frame reduces the effectiveness of the cavity somewhat. These details also usually require numerous metal fasteners (screws) which short circuit the assembly and provide a path of least resistance for thermal conductance, accounting for a not insignificant amount of heat transfer. Some current systems also provide field assembled thermally improved framing systems which rely on the skill of the installers to maintain the integrity of the system.
Therefore, there is a need in the art for an improved green building technology that securely attaches an exterior wall system to structural framing while greatly reducing heat transfer through the thermal weak points, materials with high thermal conductivity such as metal.